function Map(el){ //el形参，接收调用时传过来的盒模型
    this.el=el
    this.width=800
    this.height=400
}
Map.prototype.create=function(){   //增加一个原型方法，只不过名字叫create
    this.el.style.width=this.width+'px'
    this.el.style.height=this.height+'px'
    this.el.style.background='url(./bg.png)'
    this.el.style.border='10px solid green'
    this.el.style.position='relative'
}